InputStream from a URL

Question

How do I get an InputStream from a URL?

for example, I want to take the file at the url wwww.somewebsite.com/a.txt and read it as an InputStream in Java

Answer 1

Pure Java:

 urlToInputStream(url,httpHeaders);

With some success I use this method. It handles redirects and one can pass a variable number of HTTP headers asMap. It also allows redirects from HTTP to HTTPS.

private InputStream urlToInputStream(URL url, Map args) {
    HttpURLConnection con = null;
    InputStream inputStream = null;
    try {
        con = (HttpURLConnection) url.openConnection();
        con.setConnectTimeout(15000);
        con.setReadTimeout(15000);
        if (args != null) {
            for (Entry e : args.entrySet()) {
                con.setRequestProperty(e.getKey(), e.getValue());
            }
        }
        con.connect();
        int responseCode = con.getResponseCode();
        /* By default the connection will follow redirects. The following
         * block is only entered if the implementation of HttpURLConnection
         * does not perform the redirect. The exact behavior depends to 
         * the actual implementation (e.g. sun.net).
         * !!! Attention: This block allows the connection to 
         * switch protocols (e.g. HTTP to HTTPS), which is not 
         * default behavior. See: https://stackoverflow.com/questions/1884230 
         * for more info!!!
         */
        if (responseCode < 400 && responseCode > 299) {
            String redirectUrl = con.getHeaderField("Location");
            try {
                URL newUrl = new URL(redirectUrl);
                return urlToInputStream(newUrl, args);
            } catch (MalformedURLException e) {
                URL newUrl = new URL(url.getProtocol() + "://" + url.getHost() + redirectUrl);
                return urlToInputStream(newUrl, args);
            }
        }
        /*!!!!!*/

        inputStream = con.getInputStream();
        return inputStream;
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

Full example call

private InputStream getInputStreamFromUrl(URL url, String user, String passwd) throws IOException {
        String encoded = Base64.getEncoder().encodeToString((user + ":" + passwd).getBytes(StandardCharsets.UTF_8));
        Map httpHeaders=new Map<>();
        httpHeaders.put("Accept", "application/json");
        httpHeaders.put("User-Agent", "myApplication");
        httpHeaders.put("Authorization", "Basic " + encoded);
        return urlToInputStream(url,httpHeaders);
    }